Résumé
One of the challenges in the energy transition is minimization of the battery cost for energy grid storage. Na‐ion batteries are a promising alternative to Li‐based analogues thanks to low cost, abundance of constituents, and an already set‐up industry. However, already commercialized positive electrode materials for Na‐ion batteries (Na 3 V 2 (PO 4 ) 2 F 3 and Na x Cu 1‐y‐z Fe y Mn z O 2 ) contain critical raw elements such as V, Cu, and Mn, boosting the research towards abundant Fe‐based materials. In this work, we report a novel sodium iron sulfate phase, Na 2 Fe 3 (SO 4 ) 4 (NFS), synthesized by ball milling, as a positive electrode material. This new compound crystallizes in the orthorhombic Pbca space group with cell parameters a = 9.682(1) Å, b = 8.739(1) Å, c = 29.300(4) Å, and V = 2479.0(5) Å 3 . Tests of thick NFS positive electrodes (18 mg cm −2 ) in Na‐half coin cells delivered 62 mAh g −1 at C/30 (corresponding to the exchange of 2 e‐) or 69% of the theoretical capacity (90 mAh g −1 ), with a good capacity retention of 47 mAh g −1 at a high discharge rate 2C. Operando X‐ray diffraction (XRD) showed minimal (<1%) changes in lattice parameters during cycling. Dominantly octahedra coordinated iron atoms are oxidized/reduced, as confirmed by operando synchrotron Mössbauer spectroscopy.